Whenalabyrinthisirritatedinoneear,asinMeniere’s disease, orwhenalabyrinthisrenderednonfunctional,asmayhappenasaresultofheadtraumaordiseaseofthelabyrinth,thesignalstransmittedthroughtheVORpathwaysfromthetwosidesbecomeunbalanced.Vestibularnystagmuscanthenresult.Forexample,irritationofthelabyrinthoftheleftearcanincreasethedischargeofafferentfibersthatsupplythelefthorizontalsemicircularduct.Thesignalproducedresemblesthatnormallygeneratedwhentheheadisrotatedtotheleft.Becausethestimulusisongoing,aleftnystagmusresults,withaslowphasetotheright(causedbytheVORpathway)andafastphasetotheleft.Destructionofthelabyrinthintherightearproduceseffectssimilartothoseinducedbyirritationoftheleftlabyrinth.Interestingly,thenystagmusistemporary,whichshowstheabilityofthesecircuitstoadaptovertime.
Circuits Underlying the Optokinetic Reflex
The stimulus eliciting the OKR is visual (retinal slip), and so photoreceptors are the start of the reflex arc. Key brainstem centers for this reflex lie in the tegmentum and pretectal region of the rostral midbrain. They are the nucleus of the optic tract (NOT) and a group of nuclei collectively known as the accessory optic nuclei (AON). Direction-selective, motion-sensitive retinal ganglion cells are a major afferent source carrying visual information to these nuclei. In addition, input comes from primary and higher order visual cortical areas in the occipital and temporal lobes. These latter afferent sources are particularly important in primates and humans. Cells of the NOT and AON have large receptive fields, and their responses are selective for the direction and speed of movement of the visual scene. Of interest is that the preferred directions of movement of the NOT/AON cells correspond closely to motion caused by rotation about axes perpendicular to the semicircular canals, thereby facilitating coordination of the VOR and OKR to produce stable retinal images.
The efferent connections of these nuclei are numerous and complex and not fully understood. There are polysynaptic pathways to the oculomotor and abducens nuclei and monosynaptic input to the vestibular nuclei, which allow interaction with the VOR. There are projections to various precerebellar nuclei, including the inferior olivary nucleus and basilar pontine nuclei. These pathways then loop through the flocculus and back to the vestibular nuclei. In sum, via several pathways operating in parallel, activity ultimately arrives at the various oculomotor nuclei whose motor neurons are activated, and proper counterrotation of the eyes results.
Saccades are generated in response to activity in the superior colliculus or the cerebral cortex (frontal eye fields and
ClinicaltestingoflabyrinthinefunctionisdoneeitherbyrotatingthepatientinaBáránychairtoactivatethelabyrinthsinbothearsorbyintroducingcoldorwarmwaterintotheexternalauditorycanalofoneear(caloric test). WhenapersonisrotatedinaBáránychair,nystagmusdevelopsduringtherotation.Thedirectionofthefastphaseofthenystagmusisinthesamedirectionastherotation.Whentherotationofthechairishalted,nystagmusintheoppositedirectiondevelops(postrotatorynystagmus)becausestoppingarotationhasthesameeffectasacceleratingintheoppositedirection.